Phthalic anhydride (PA) is widely used in industry in the production of dyes (rhodamine, anthraquinone derivatives), insecticides, plasticizers, in pharmacy, and in analytic chemistry. PA is conventionally prepared industrially by catalytic gas-phase oxidation of orthoxylene or naphthalene in shell-and-tube reactors. The starting material is a mixture of a gas comprising oxygen, such as air, and the orthoxylene and/or naphthalene to be oxidized. The mixture is passed through a multiplicity of tubes arranged in a reactor (shell-and-tube reactor), in each of which a bed of at least one catalyst is located. Known PA catalysts include vanadium oxide and titanium oxide as active ingredients supported on an inert carrier. The production of PA from orthoxylene and naphthalene is cheaper compared to other processes for forming PA, and selling extra PA product is generally not a limitation.
An exemplary conventional PA production system (“PA unit”) is described below which comprises air blower (G-11) and air preheater (E-11) section (see FIG. 1; described below), an orthoxylene feed section (see FIG. 2; described below), a naphthalene feed section (See FIG. 3; described below), and a catalytic oxidation reactor section including D-14 oxidizing reactor (see FIG. 4; described below). The exemplary PA unit also includes a cooling of reactor gases (E16) section and desublimation of PA section (E-18A, B and C, D; (see FIG. 5; described below), and catalytic incineration of waste gases (R111) section (see FIG. 6; described below).
Referring now to FIG. 1, an air blower and preheater section 100 is depicted. Turbo-blower G-11 sucks and compresses the amount of oxidizing air necessary for the reactor D-14 via air filter F-11. The air is heated up in the steam-heated air pre-heater E-11 to a temperature of approximately 160° C. The air flow measurements on the flows going to the evaporators and to the reactor are doubled on both lines. In a conventional case, doubling is used as shown in FIG. 1. The air-cooling capacity is generally not a limitation. The incoming air flow valve is adjusted to maintain a wanted air flow through F-14E air-orthoxylene mixer (shown and described in FIG. 2). The blower current is generally a very important variable for the operators and is used and an indication of the turbo blower load. After the pre-heater E-11, the flow of air is distributed into 4 branches, D-14B, D-14C, E-12B and E-12C.
FIG. 2 show an orthoxylene feed section 200. The two main air ducts convey the oxidizing air into mixers F-14D and E. In this place orthoxylene is injected into the flow of air. The generated mixture of air and orthoxylene vapors is conveyed into gas mixers F-14 B and C, respectively. The main flow mixtures of air are mixed there with the air saturated with naphthalene flumes after having passed through evaporators E-12 B and C (shown in FIG. 3, described below). The obtained mixture of air, naphthalene and orthoxylene fumes is then conveyed into the oxidizing reactor D-14, shown in FIG. 4 as reactor 400. The incoming orthoxylene flow is controlled according to the orthoxylene concentration after F-14D and E, the units generally being in g/Nm3.
FIG. 3 shows a naphthalene feed section 300. A small portion of air is aspirated by auxiliary turbo-blower G-13 and it is conveyed as carrier gas through naphthalene evaporators E-12 B and C where it is saturated with naphthalene. The required concentration of naphthalene is set according to the temperature of air at the outlet, which is regulated to the required temperature by means of the heating steam. This concentration generally exceeds the upper explosion limit.
The concentrated mixture of naphthalene and air is mixed in gas mixers F-14 B and C with the main flow of air containing orthoxylene and thus the concentration is set to the required value above the lower explosion limit of the mixture (e.g. 45 g/Nm3) TN-OX in the air. To admit orthoxylene, two orthoxylene, air mixers F-14 D and E (as shown in FIG. 2) are installed after the intake of the partial airflow for the naphthalene evaporators and ahead of gas mixer F-14 B and C. In these apparatus, the oxidizing air is enriched with orthoxylene injected under pressure by spray nozzle. For safety reasons, the upper explosion limit is set a little under the lower explosion limit.
The naphthalene is continuously pumped from a naphthalene storage tank (not shown) by one a centrifugal pumps (not shown) via one of two filters to naphthalene evaporators E-12 B and C. The still undivided stream of naphthalene passes through a tubular heat exchanger (not shown) where it is preheated by steam to the required temperature. Following the distribution, each partial flow is controlled by the regulator according to the level in the naphthalene evaporator. Orthoxylene is continuously conveyed from an orthoxylene tank (not shown) by a centrifugal pumps (not shown) via double filter F-13 B and C into preheaters E-12 D and E and then into orthoxylen—air mixers F-14 D and E. Each partial flow is controlled depending on the actual amount of the oxidizing air. After leaving mixers F-14B and C, both streams of main air are put together and mixed in the static mixer.
Every tank of the naphthalene and the orthoxylene is analyzed, generally about once per week. The quantity of the naphthalene incoming flow is set by the E-12B and C levels, but there is no measurement, only a sideglass. F-13C cyclone can be overloaded, and if so, the temperature has to be increased. The evaporators outlet temperatures (TCxxB and C) are very important and are generally controlled adequately by the steam.
The incoming air to the evaporators (and thus the total air flow) is controlled according to a single parameter being the naphthalene concentration after F-13 B and C. The respective set points are shown as NaB[ ] and NaC[ ] in FIG. 3. The total aromatic concentration before F-14B and C should be nearly equal. After mixing the B and C lines, the concentrations are: FKNAA2 for naphtha, FKOXA2 for orthoxylene and FKNOA2 for the summary of the two. This last value should not exceed about 85 g/m3, but the limit depends on the ratio of naphthalene to orthoxylene.
Referring to catalytic oxidation reactor 400 shown in FIG. 4, in the standard operation, the mixture of naphthalene and orthoxylene vapors and air within the explosion limits occurs under the upper reactor cover, in the mixers and, in the starting operation, for a limited period of time, also in the naphthalene evaporators. The naphthalene and orthoxylene mixture enters the reactor D-14 from above and exits from below as shown by the arrows in FIG. 4. The reactor conventionally includes about 14,000 vertical pipes connected in parallel which are about 3.7 m. long, which are filled with a highly efficient 4-layer oxidation catalyst. The tubes are surrounded by salt bath, generally comprising a eutectic mixture of potassium nitrate and sodium nitrite, which is continuously re-circulated by pump G-14. The mixture of naphthalene and orthoxylene vapors entering the rector at a temperature of 145 to 150° C. is first heated up to the heat of reaction by the molten salt. At a temperature of 360 to 390° C., naphthalene and orthoxylene are partially catalytically oxidized by the atmospheric oxygen mainly to the desired PA product.
A smaller portion of naphthalene is at the same time converted to 1,4maleic anhydride, or it is completely oxidized (to carbon dioxide and water). If the heat of reaction is too low, a greater amount of 1,4naphtaquinone is produced. If the temperature is too high, the proportion of maleic anhydride increases and the major part of naphthalene is completely oxidized. A part of orthoxylene is also converted to maleic anhydride or it undergoes complete oxidation. As a by-product of partial orthoxylene oxidation, phthalate is produced.
The oxidation reactions catalyzed by the catalyst are very exothermic. By means of suitable in-built structures and by recirculating salt bath G-14, the temperature in the reactor is distributed in a uniform manner. The heat of reaction is removed from the salt bath by the evaporation of condensate in evaporator E-14, where a mixture of steam and water is produced. The mixture of steam and water is conveyed into a high-pressure steam drum (not shown), where it is separated into saturated steam and condensate.
The pressure in the reactor cooling system is maintained stable by means of a controlling valve set at a value higher than the usual pressure in the plant system. The hot spot profile can change by changing the concentration. Temperatures at various positions along the height of reactor 400 are provided as single point temperature in the catalyst “layers” TID142-TID148. The highest temperature position should be between about 150-200 cm from the reactor top, it can be controlled by controlling the TID148 temperature that should generally never go above 420 C. The speed of temperature changing is also generally important, but it is not critical.
The temperature of salt bath G-14 temperature can generally vary ±0.25° C. in stable operation. By increasing the salt bath temperature the catalyst temperature will decrease, by decreasing the salt bath temperature, the catalyst temperature will increase. This is a direct way of controlling the highest catalyst temperature. Changing of the salt bath temperature does not change the production, only the lifetime of the catalyst.
The stream of the reaction gas let out from the lower part of reactor D-14 is conveyed into the common housing of two-stage cooler E-16, where the heat contained in the gas is used for the generation of steam. The cooler proper comprises four sections of vertical finned tubes. The first two sections are connected by means of piping with a steam drum (not shown) and the remaining sections with another steam drum (not shown).
In the first two bundles, the boiler water from drum changes into a mixture of steam and water and due to the thermo-siphon effect, it is conveyed back to the drum, in which the steam is separated and conveyed for further use. In a similar manner, the third and the fourth cooler bundles are interconnected with a drum.
FIG. 5 shows a desublimation of phthalic anhydride section 500. The reaction gas gradually cooled down in cooler E-16 is further cooled in desublimators E-18 A-D. The desublimator comprises four heating bodies consisting of finned tubes located in a common housing. The reaction gas enters the desublimator from above and it is cooled down. In the course of the cooling process, phthalic anhydride is deposited on the fins of the tubes in the form of rod-like crystals with an efficiency of up to 99.5%. After the phthalic anhydride is isolated, the reaction gas turns into waste gas, which is conveyed to catalytic incinerator for final purification.
FIG. 6 shows a catalytic incineration of waste gases section 600. The flow of gases from desublimators E-18A through D shown in FIG. 5 contains residues of organic matter which have not been isolated (PA, maleic anhydride, etc.), carbon monoxide and carbon dioxide. These substances, in addition to the already present carbon dioxide, have to be catalytically incinerated to obtain carbon dioxide and water. Thus, undesirable emission of pollutants is prevented when waste gases are exhausted from the phthalic anhydride production plant to the atmosphere.
The waste gases are conveyed from the outlet of desublimators E 18A through D to the catalytic incinerator 600. The waste gas is first conveyed to steam pre-heater E-111 in which it is heated up by means of tubular heat exchanger heated up by steam. The waste gas then flows inside the heat exchanger E-112 tubes where it is pre-heated by counter-current by the clean waste gas to a temperature sufficient for the combustion function of the catalyst. The hot waste gas is then conveyed from heat exchanger E-112 to reactor R-111 fitted with a platinum catalyst in two levels on a ceramic carrier.
On the inner surface of the ceramic carrier bricks provided with a layer of platinum catalyst, all the organic substances and carbon monoxide will be incinerated. In the course of incineration, heat is released and thus the temperature of waste gases increases. The uncontaminated waste gas is exhausted to chimney C-81 via heat exchanger E-112.
If necessary (when the raw materials throughput in reactor D-14 is low), the temperature of waste gases in front of the heat exchanger E-112 will be increased in such a manner that it be sufficient for preheating the untreated waste gas before its entry on the catalyst. A part of the uncontaminated waste gas is therefore conducted to gas furnace C-111, where the waste gas is mixed with hot combustion gases of the gas burner.
If the supply of combustible substances is very high, the danger of catalyst overheating in reactor R-111 is imminent and if the heat transmission in heat exchanger E-112 is reduced, chimney C-81 may be overheated. In such a case, the autothermic operating mode takes place and no earth-gas is burnt down in furnace C-111. It is generally necessary to ensure continuous operation of a fan of the air of combustion (not shown) to prevent the corrosion of colder parts of the furnace by sulphur oxides.
The catalytic incinerator of waste gases is also used to decontaminate other gaseous emissions, such as from technical naphthalene, orthoxylene, naphthalene residues, pure phthalic anhydride complete with the filling condition of the truck tanks, exhaustion of the tank for the discharge of the distillation residue and the supply of waste gases from the adjacent naphthalene production plant.
Maximization in the PA production process refers to the attempt to get more PA out of the PA unit. Throughput for the PA unit is conventionally maximized using a process controller subject to a plurality of maximization constraints including a maximum air compressor flow, a maximum reactant concentration at the oxidation reactor inlet, a maximum orthoxylene concentration below low explosive level in the feeding section, a maximum naphthalene concentration above high explosive level in the feeding section, a maximum oxidation reactor catalyst temperature, and a maximum waste gas catalytic incinerator differential temperature (outlet-inlet). All of these maximization constraints are limitation factors for the conventional PA unit. As a result, once the first of the plurality of constraints become active (are reached), the maximization (attempt to get more PA out of the PA unit) process is stopped.
Moreover, present control methodologies implemented by commercially available process controllers generally manipulate only a single manipulated variable (MV) to control another variable, referred to as a controlled variable (CV). As noted above, for example, the incoming air to the evaporators, and thus the total air flow, is a CV that is controlled according to only the naphthalene concentration after F-13 B and C, a single MV (see FIG. 3). Conventional single variable control methodologies thus limit the obtainable conversion efficiency into PA product. What is needed for improving conversion efficiency is a multivariable controller that manipulates a set of MVs to maintain a set of CVs within constraints or targets. Such a multivariable control methodology would allow PA unit maximization to continue after the first constraint activation occurs.